Transforming growth factor β induces caspase 3-independent cleavage of αII-spectrin (α-fodrin) coincident with apoptosis

Transforming growth factor beta (TGF-beta) is a potent growth inhibitor and inducer of cell death in B-lymphocytes and is essential for immune regulation and maintenance of self-tolerance. In this report the mouse immature B cell line, WEHI 231, was used to examine the mechanisms involved in TGF-beta-mediated apoptosis. Induction of apoptosis is detected as early as 8 h after TGF-P administration. Coincident with the onset of apoptosis, the cytoskeletal actin-binding protein, alpha II-spectrin (cy-fodrin) is cleaved into 150-, 115-, and 110-kDa fragments. The broad spectrum caspase inhibitor (Boc-D-fmk (BD-fmk)) completely abolished TGF-P-induced apoptosis and alpha II-spectrin cleavage. Caspase 3, although present in WEH1 231 cells, was not activated by TGF-P, nor was its substrate, poly(ADP-ribose) polymerase. These results identify alpha II-spectrin as a novel substrate that is cleaved during TGF-P-induced apoptosis. Our data provide the first evidence of calpain and caspase 3-independent cleavage of alpha II-spectrin during apoptosis and suggests that TG;F-P induces apoptosis and alpha II-spectrin cleavage via a potentially novel caspase. This report also provides the first direct evidence of caspase 3 activation in WEH1 231 cells and indicates that at least two distinct apoptotic pathways exist.